The present invention relates to a device for determining a coordinate of a point on a flat surface, and more particularly to a device for determining where an arbitrary point of a graphic material is located on an X-Y coordinate system by using electromagnetic induction.
A coordinate position determining device is one of generally so-called digitizers, wherein a coordinate position of an arbitrary point on a flat surface is determined in terms of phase shift amount by using electromagnetic induction, and the coordinate position is outputted in the form of digital signal. Most of the devices of this type have adopted a two-phase grid system.
As a typical example of such prior art, there is known a Japanese Patent Laid-open Publication JP-A-49-67521 (Japanese Patent Publication JP-B-53-34855) which employs a coordinate determining tablet of three layers with an X-axis base, Y-axis base and selector base laminated from the top in this order. The X-axis base comprises first and second conducting wires formed on one surface of an electrically insulating sheet. The first conducting wire is configured to have a zig-zag pattern composed of a plurality of rectangles with one empty side alternately disposed at every half pitch along the X-axis, while the second conductive wire, although it is formed to have a same continuous zig-zag pattern as the first conductive wire, is electrically insulated and displaced from the first conductive wire by a quarter pitch in the X-direction. Similarly, the Y-axis base comprises two conductive wires formed on one surface of an electrically insulating sheet, the two conductive wires forming two continuous zig-zag patterns with a quarter pitch displaced from each other in the Y-direction. The selector base comprises two sets of conductive wire elements, one set of conducting wire elements disposed in parallel along the X-direction and being insulated from and intersecting at right angles to the other set of conducting wire elements disposed in parallel along the Y-direction. The three bases are laminated to form the coordinate determining tablet. As a cursor having a coil applied with an AC reference signal moves on the coordinate determining tablet, a phase modulated signal corresponding in amount to a displacement of the cursor is induced, at a cycle of interest, to parallel portions of conductive wires of the X-axis base for example. The displacement of the cursor can be determined over one cycle (360 degrees in electrical angle) by comparing the phase modulated signal with the phase of the AC reference signal applied to the cursor. In this case, assuming that only the two conducting wires displaced by a quarter pitch from each other are used, it cannot be identified if the cursor position corresponds to a position of an electrical angle .theta. or that of an electrical angle of n.times.360.degree.+.theta. (where n is a positive integer). However, since the above prior art utilizes the selector base having conductive wire elements disposed in parallel, a output voltage having its magnitude corresponding to a different electrical angle is generated in accordance with the cursor position between two adjacent conductive wire elements. By detecting this output voltage, it becomes possible to determine the cursor position at a correct cycle, The cursor position relative to the Y-axis can be determined in a similar manner. The above-described prior art is excellent except a use of the selector base which results in a complicated structure of the tablet. In addition, since the structure of laminated three bases is used, the accuracy of determining the cursor position detends greatly not only upon the precision of arrangement of conductive wires on each base but also upon the precisions of alignment and lamination of the bases. Moreover, because of the above reason, the number of manufacturing processes of the apparatus and hence the cost may increase.
The disadvantages of the above Japanese Patent Laid-open Publication JP-A-49-67521 have been solved by Japanese Utility Model Laid-open Publication No. 55-55709 (Japanese Utility Model Publication No. 59-14858) as in the following manner. Namely, the coordinate determining tablet is constructed of X-axis cursor position determining conductive wires and Y-axis cursor position determining conductive wires, respectively formed on one and the other surfaces of a single electrically insulating sheet and intersected at right angles. The X- and Y-axis cursor position determining wires each are electrically insulated and arranged to have a comb configuration displaced by 180 degrees in electrical angle. The coordinate determining tablet according to the Japanese Utility Model Laid-open Publication No. 55-55709 is manufactured using a single sheet at minimum (or two sheets if one surface per sheet is used). Therefore, this tablet has various advantages in practical use, over the tablet disclosed in the first-mentioned Japanese Patent Laid-open Publication JP-A-49-67521 which requires a selector base. Namely, the manufacturing process is simple, the cost is low, there is no fear of a low alignment precision, and so on. The following disadvantages have been found, however, in the Japanese Utility Model Laid-open Publication No. 55-55709:
(a) Conductive wires selectively used for detecting a phase difference cover only half the pitch and in addition, the tablet is constructed of simply a two-phase system (sin, cos). Therefore, not only the range of change in phase difference is 180 degrees, but also a change in position of a position determining cursor becomes disproportional to a corresponding phase change near the conductive wires, resulting in a large distortion.
(b) Since detected voltages by the conductive wires are compared with each other to identify a specific location where the position determining cursor is present, the position detection becomes difficult in the case where the position determining cursor departs even slightly from the conductive wires, i.e., fluctuates up or down relative to the conductive wires.
Instead of tablets adopting a two-phase grid system as in the above-described prior art, there are known other tablets adopting a three-phase grid system as described in U.S. Pat. Nos. 4,552,991 and 4,570,033. The reason of adopting the three-phase grid system in these U.S. Pat.'s is to eliminate so-called edge effect. Particularly, in the case of the Japanese Patent Laid-open Publication JP-A-49-67512, conductive wires on the X-axis base juxtaposed in parallel with the Y-axis are sequentially coupled together by end turns in parallel with the X-axis to form a single continuous zig-zag pattern as a whole. However, a voltage induced to the end turn by the exciting cursor coil affects an output signal voltage appearing across the output terminals of the conductive wire of the X-axis base, irrespective of the fact that the induced voltage is not related to the displacement of the cursor along the X-axis. Thus an error occurs. As a means for eliminating such an error caused by the edge effect, it can be considered not to use the tablet area which is likely to produce an edge effect, which essentially makes the effective area of the tablet smaller and causes inconvenience in practical use. Further, an alternative method of eliminating such an error by using a suitable electronic circuit or by software may cause a complicated structure and operation of the tablet. In view of this, according to the above-mentioned U.S. Pat.'s, each set of the conductive grid windings of the X-and Y-axis bases are constructed of a three or more odd phase structure to thus compensate for error voltages induced to the end turns. Consequently, the position determining accuracy of the above-mentioned U.S. Pat.'s is improved as compared with that of a two-phase system. However, the following disadvantages are still present:
First, the structure and operation of the coordinate determining tablet not only of the type which adopts the system disclosed in U.S. Pat. Nos. 4,552,991 and 4,570,033 but also of the other type can be generally shown as in FIGS. 1 and 2, respectively. Since the conductive wire becomes asymmetrical relative to the cursor coil at both end portions thereof as seen from FIG. 2, there arises a disadvantage that a position determining error is likely to occur at both the end portions.
Second, the tablet disclosed in U.S. Pat. Nos. 4,552,991 and 4,570,033 requires in practice a first conductive wire group composed of three wires each having a pitch L as shown in FIG. 2 and a second conductive wire group (not shown in FIG. 2) composed of three wires each having a pitch L+.DELTA.l. Therefore, the tablet for both the directions of X-axis and Y-axis requires four conductive wire groups (systems), thus resulting in cumbersome and difficult pattern formation.
Furthermore, according to the above-mentioned U.S. Pat.'s, the cursor position along the X-axis is detected in a manner as shown in FIG. 3 wherein only the conductive wire group in the X-axis direction is shown. Specifically, as shown in FIG. 3(1), it is possible to obtain a phase angle .theta..sub.1 from the conductive wire group having a pitch L and a phase angle .theta..sub.2 from the conductive wire group having a pitch L+.DELTA.l, by detecting a phase difference from a reference phase of the excited cursor. The curve shown in FIG. 3(2) represents a phase difference .theta..sub.1 -.theta..sub.2 between the two phase angles .theta..sub.1 and .theta..sub.2. By adding 360 degrees to the negative portions (indicated by triangles) of the curve shown in FIG. 3(2), a linear phase angle line .theta..sub.0 can be obtained as shown in FIG. 3(3).
If the center of the cursor is positioned at point A shown in FIG. 3(3), it can be identified that the center is located at the fourth cycle of the conductive wire group having a pitch L (coarse position determination or cycle number determination). By detecting the magnitude of the phase angle .theta..sub.1 at the fourth cycle of the conductive wire group having a pitch L, it is possible to correctly determine the position (fine position determination).
In manufacturing a large scale coordinate determining tablet of the above-mentioned U.S.P.'s, it is necessary to make the pitch difference .DELTA.l of the two conductive wire groups small. This is due to the fact that a large .DELTA.l restricts a span of the X-axis as the phase angle .theta..sub.0 shown in FIG. 3(3) soon becomes 360 degrees.
On the contrary, if the .DELTA.l is set too small, the incline of .theta..sub.0 shown in FIG. 3(3) becomes so small as to be unable to ensure a sufficient accuracy of coarse position determination. To retain a good accuracy, it is necessary to make the pitch of the first conductive wire group large, which results in a lower resolution.